The present invention relates to d drive circuit for switching device and a semiconductor driver circuit of a normally-off junction FET, and a semiconductor device using the same.
A wide band-gap semiconductor element, which employs a wide band-gap material such as silicon carbide (SiC), gallium nitride (GaN), or diamond, exhibits excellent characteristics as a switching element. Here, examples of such a wide band-gap semiconductor element are as follows: Junction FET (JFET), Static Induction Transistor (SIT), MEtal-Semiconductor Field Effect Transistor (MESFET), Hetero junction Field Effect Transistor (HFET), and High Electron Mobility Transistor (HEMT). Also, in the junction FET having normally-off characteristics, even if a malfunction of a circuit has occurred, e.g., even if a gate driver circuit has failed, the short-circuit and failure can be prevented.
The normally-off junction FET is represented by and is equivalent to an equivalent circuit in which a parasite diode is connected in parallel to an input capacitor between the gate and the source. On account of this fact, in the case of the junction FET which employs SiC, the threshold value for turning on the switching element becomes equal to about 2.5 V, which is a significantly low value. Accordingly, in the normally-off junction FET, there have existed problems such as high-accuracy voltage control, high-speed charging into the input capacitor, and misoperations.
The inter-gate-source characteristics of the junction FET become the forward-direction characteristics of the diode. On account of this fact, applying thereto a voltage which is higher than the rise-up voltage causes a large gate current to continue flowing. As a result, power consumption by the gate driver circuit has increased.
Also, the threshold value is low, and thus the large gate voltage cannot be applied thereto at the time of turn-on. On account of this fact, the input capacitor cannot be charged at a high speed. As a result, the turn-on loss has increased.
Also, the noise-resistant amount is small, and thus a possibility of giving rise to occurrence of the misoperations is high. As a result, there have occurred, e.g., the self turn-on phenomenon and a misoperation at the time of turn-off.
Moreover, in the conventional technologies disclosed in JP-A-2000-243905 and JP-A-2004-14547, the proposal has been made concerning a technique for preventing the misoperation due to the self turn-on phenomenon by connecting a capacitor between the gate and the source. No description, however, has been given regarding the misoperation at the time of turn-off. Also, the conventional technologies relate to a driver circuit that uses an IGBT (Insulated-Gate Bipolar Transistor) whose threshold value is higher as compared with the normally-off junction FET. Namely, no description has been given concerning a circuit for driving a low-threshold-value element with a low loss.